What is photosensitivity?
Photosensitivity is a sensitivity to sunlight or ultraviolet (UV) light, and can sometimes increase your risk of sunburn, depending upon the cause. In some people, photosensitivity can cause a skin rash or other type of skin lesion, and skin lesions triggered by sunlight or UV light are called photodermatoses.
What are photoexacerbated dermatoses?
Photoexacerbated dermatoses, sometimes referred to as photoaggravated dermatoses, are a form of photodermatoses caused by a pre-existing condition or skin disease. They can get worse on exposure to sunlight or UV light, normally UVA and not UVB light (Cunliffe, 2016).
UVB light is short-wave light that causes skin reddening and sunburn. UVA is long-wave light and penetrates the skin more deeply than UVB, causing skin ageing and wrinkling. It also damages cells called keratinocytes in the bottom layer of the epidermis, the outer layer of the skin, contributing to the development of skin cancer (Skin Cancer Foundation, 2013).
Photoexacerbated dermatoses include:
- Lupus erythematosus (Oakley, 2016).
- Darier disease.
- Pemphigus vulgaris.
- Pemphigus foliaceus.
- Atopic dermatitis.
Is Sweet’s syndrome a type of photoexacerbated dermatosis?
In 2011, Sweet’s syndrome was experimentally induced in a 78-year-old man by exposing him to UV light, both UVA and UVB, for three days in a row (Meyer et al, 2011). In 2014, Sweet’s syndrome lesions developed on the arms and back of a 40-year-old woman who had been working in the sun (Verma et al, 2014). It was previously thought that Sweet’s syndrome was a photoexacerbated dermatosis, but ongoing research has suggested that it probably isn’t.
Why can sunlight or UV light trigger Sweet’s syndrome?
As yet, it is not fully understood why sunlight or UV light can sometimes trigger Sweet’s syndrome. However, there are two main theories as to why this might happen.
THEORY NO. 1.
Koebner phenomenon or isomorphic response (Meyer et al, 2011; Verma et al, 2014). Read more here.
What does this mean?
Koebner phenomenon or isomorphic response is a phenomenon that causes skin lesions to appear at the site of a skin injury in otherwise healthy skin. It can happen for a number of different reasons, including overexposure to sunlight and phototherapy (light treatment).
THEORY NO. 2.
Pathergy response, or overexposure to UVB light leading to the activation of white blood cells called neutrophils as a result of cytokine production (Meyer et al, 2011; Verma et al, 2014).
What does this mean?
This means that when the cells of the skin are damaged by sun, cells next to the damaged cells start spreading cytokines. These are proteins and molecular messengers that are stored by the cells of the immune system. When these cytokines are released, this causes inflammation as the first response of the immune system to destroy the damaged cells after which the damaged skin starts to heal. Certain cytokines have been proven to play a role in Sweet’s syndrome, including endogenous granulocyte colony-stimulating factor (G-CSF), which increases the number of neutrophils in the blood. Read more here.
Dapsone and tetracycline antibiotics, e.g. minocycline and doxycycline, are treatments for Sweet’s syndrome. They are known to cause photosensitivity, and this can increase the risk of sunburn.
Cunliffe, T. (2016) Photodermatoses: on overview. PCDS: Primary Care Dermatology Society (online). Created 9th Oct 2011, and updated 6th July 2016. Accessed 28/06/17.
Oakley, A. (2016) Photosensitivity. DermNet NZ (online). Originally published in 1997, and updated by Professor Oakley, Jan 2016. Accessed 28/06/17.
Skin Cancer Foundation (2013) UVA & UVB (online). Medical reviewers, John H. Epstein, MD, and Stephen Q. Wang, MD. Accessed 28/06/17.
Ngan, V. (2006) Drug-induced photosensitivity. DermNet NZ (online). Accessed 28/06/17.
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